With RGB...I grab the center of the curve and swing it back and fourth just a little to get the feel for what the picture needs....once I get it looking better...then I play with both sides of that center point to see where they need to go....and so on....each channel.

CMYK...I usually have to grab the lower left hand side and slid it in a bit to get to the edge of the color information....it will look ugly a bit at first...then do just like the RGB....grab a little bit more to the left....till it starts to straighten out...then flatten the rest of the line up to the upper right giving little bitty fine tunings till you get all the way up to the right....each color channel...if the channel fills the base...just treat it like an RGB stated above.

LAB....I adjust per the basics of Margulis's technique....I bring the layer down to about 15% then start sliding the edges in....in A&B mode. (after trating the L curve like a regular RGB curve.

Hope this helps.

Roman